Urethanes can be catalytically or thermally decomposed to form corresponding isocyanates, which are important key materials in the polyurethane (PU) industry as well as in the pharmaceutical industry. In the prior art, there are two major methods disclosed by various patents for urethane preparation.
One method is reductive carbonylation, which can be represented by the following general reaction equation: EQU R(NO.sub.2).sub.x +3xCO+xR'OH.fwdarw.R(NHCOOR').sub.x +2xCO.sub.2 (I)
Another method is oxidative carbonylation, which can be represented by the following general reaction equation: EQU R(NH.sub.2).sub.x +xCO+xR'OH+x/2O.sub.2 .fwdarw.R(NHCOOR').sub.x +xH.sub.2 O (II)
In these formulas, R is the organic radical of the corresponding nitro or primary amino group containing compound, R' is the organic radical of the hydroxyl group containing compound, and x is an integer.
The disclosed catalyst systems for both methods mainly are group VIIIB metals of the periodic table, in combination with nitrogen or phosphorous containing compounds as promoters.
For the reductive carbonylation method, U.S. Pat. No. 4,603,216 discloses that a halide-free ruthenium complex catalyst comprising a bis-phosphine ligand was employed to catalyze the reaction. Also, U.S. Pat. No. 3,338,956 describes a process for the manufacture of urethanes at high temperature and high pressure in the presence of a metal carbonyl of elements in groups VIB, VIIB and VIIIB of the periodic table, while U.S. Pat. No. 3,998,685 describes a process where palladium halides and ferric chloride were used. See, also: S. Oenini, et al, J. Chem. Soc., Chem. Commun., 1286, 1984, in which ruthenium carbonyl catalyst was used to prepare urethanes with high selectivity in the presence of alkyl ammonium salt; and Y. Watanabe, et al, Bull. Chem. Soc. Jpn., 56: 3343, 1983, which demonstrated that a series of nitroarenes was transformed into corresponding urethanes in moderate to excellent yields with a platinum complex as catalyst.
For the oxidative carbonylation method, U.S. Pat. No. 4,297,501 discloses a catalyst system where halides of a noble metal of the VIIIB group of the periodic table, and a compound capable of undergoing redox reactions were employed. Also, European Pat. Appl. 083096 (published 1983) discloses a process for preparing urethane in the presence of a catalyst system comprising the Pt group metals and halides of alkali metals or alkaline earth metals.
Of course, the fact that the above mentioned patent teachings use expensive noble metal catalysts diminishes the commercial attractiveness of these known processes.